1. Field of the Invention
The invention relates in general to a safety circuit for a data processing system emitting binary signals employing an amplifier including an output transformer which has a secondary winding connected to a controlled element to supply fail safe signals through a rectifier circuit.
2. Description of the Prior Art
In many technical fields, electronic installations are increasingly used for the purpose of controlling automated operational cycles. Included in this category for example, are those installations which have become conventional and use process control computers and micro computers. In the field of railway safety engineering, special switching devices or sequential circuits are used with specific logic circuits which are constructed from passive and active elements such that proper functioning of these elements is required to assure the safety of the railway traffic. However, since it is inevitable that certain types of defects will occur with components, that logic circuits are generally constructed such that for the purpose of railway safety engineering errors in the components at the most have an operational impeding effect and cannot produce an effect which endangers the operation of the vehicles, for example. The result is that in utilizing modern data processing systems whose systems are not constructed according to the above given safety engineering criteria such data processing systems cannot readily be used in technical fields where the safety reliability is required as in the case of railway operating systems. This also applies uniformly in every instance where processes, in the event of a malfunction of the controlling installation must be transferred into a state which is non-dangerous to human beings and machines.
Several examples will be given to clarify this principle. Control devices in nuclear medicine under certain circumstances must be controlled so that irradiation will be interrupted.
In the operation of passenger cars, a safe radar controlled distance warning system or a safe control for a skid-free braking system can be constructed such that a defect which has occurred makes itself known to the driver before the error causes a malfunction which is hazardous to the driver's safety.
Also, in traffic light control systems, large chemical processes, street roller control systems generating plant control systems and in monitoring nuclear reactor system fail safe control is required.
For the technically safe output of binary signals from an electronic switching mechanism, generally alternating voltage amplifiers are utilized which deliver their energy for the purpose of controlling random consumer elements as, for example, relays through a transformer and a rectifier circuit which is fed by the transformer. Circuits of this type have the advantage in that random defects particularly short circuits in the switching path of an amplifier transistor always produce a reduction in the output voltage and the circuit must never produce an output voltage which connects the control relay to a signal so as to cause a false signal or non-desired process or feedback control. The output circuits given as examples here must be utilized according to the principles of safety engineering such that with the value of 0 their output voltage is put in a safe condition wherein the operational controlled element cannot be energized.